Nictitating-membrane

noun
1.
a thin membrane, or inner or third eyelid, present in many animals, capable of being drawn across the eyeball, as for protection.
/ˈnɪktɪˌteɪtɪŋ/
noun
1.
(in reptiles, birds, and some mammals) a thin fold of skin beneath the eyelid that can be drawn across the eye Also called third eyelid, haw
nictitating membrane
(nĭk’tĭ-tā’tĭng)
A transparent inner eyelid in birds, reptiles, amphibians, and some mammals that protects and moistens the eye without blocking vision.